In the recent wide use of mobile terminals represented by, for example, a mobile handset, demands are increasing for an HF semiconductor device operable at a high frequency of 1 GHz or higher. Conventionally, a gallium arsenide substrate is used for the HF frequency semiconductor device. However, the gallium arsenide substrate is expensive, so that using the gallium arsenide substrate makes it difficult in implementing reduction in cost for the HF semiconductor device. In addition, the gallium arsenide substrate makes it difficult to implement the improvement in integration of the HF semiconductor device.
In comparison to the above, while a silicon substrate with transistors formed thereon is inexpensive, there has been a difficulty in driving the transistors to sufficiently operate at an HF band. However, according to, for example, the advanced microfabrication technologies for silicon semiconductors, even HF semiconductor devices using a silicon substrate are expected to reach a level that satisfies specifications necessary for mobile terminals. It is now considered whether HF semiconductor using silicon substrate can be used for semiconductor device having high frequency of 1 GHz or more.
Among problems in mounting an HF semiconductor device on a silicon substrate is a problem of a signal loss occurring with a silicon substrate. The loss occurs because the silicon substrate is formed of an electroconductive substrate that allows a relatively high current to flow. However, since the gallium arsenide substrate is formed of a high resistant substrate, the aforementioned problem does not occur therewith.
Hereinbelow, the aforementioned signal loss will be described in detail with reference to FIGS. 16A and 16B. FIG. 16A is a plan view of a prior-art HF semiconductor device; and FIG. 16B is a cross sectional view taken along the line A-A′ of FIG. 16A.
Referring to the above-referenced drawings, numeral 901 denotes a doped electroconductive p-type silicon substrate, numeral 902 denotes an insulator layer formed of an SiO2 insulator film or the like, 903 denotes a bonding pad necessary for coupling a bonding wire, numeral 904 denotes a wire for coupling devices such as a pad and a transistor, and numeral 905 denotes a protection layer.
The bonding pad 903 and the wire 904 are each formed of, for example, one of a metal film made of, for example, Al or Cu. The insulator layer 902 covers the surface of the p-type silicon substrate 901. The bonding pad 903 and the wire 904 are formed on the surface of the insulator layer 902. The protection layer 905 is formed on the surfaces of the bonding pad 903 and the wire 904. In addition, the p-type silicon substrate 901 is connected to a ground node (not shown).
In the configuration of the prior-art HF semiconductor device shown in FIGS. 16A and 16B, an HF signal flowing through the bonding pad 903 and the wire 904 leaks to the silicon substrate 901 via parasitic capacitance of the insulator layer 902. As such, resistant components of the p-type silicon substrate 901 cause a loss by the signal leakage.
A known prior-art example HF semiconductor devices capable of reducing a loss occurring in a silicon substrate as described above is described in “A bond-pad structure for reducing effects of substrate resistance on LNA performance in a silicon bipolar technology”, Proc. 1998 IEEE BTCM.
FIGS. 17A and 17B shows another prior-art HF semiconductor device. FIG. 17A is a plan view of the HF semiconductor device; and FIG. 17B is a cross sectional view taken along the line A-A′ of FIG. 17A. Portions corresponding to those shown in FIGS. 16A and 16B are shown with the same reference numerals. Numeral 1011 denotes an n-type silicon layer having a doping concentration that is about double-digit higher than that of the p-type silicon substrate 901. Numerals 1012 and 1013 denote wires provided for coupling the n-type silicon layer 1011 to a ground potential.
The n-type silicon layer 1011 is formed on the surface of a p-type silicon substrate 901. An insulator layer 902 covers the n-type silicon layer 1011 and the p-type silicon substrate 901. However, in the covering process, terminal portions 1011a and 1011b of the n-type silicon layer 1011 are not covered and are exposed to the surface of the insulator layer 902.
A bonding pad 903 and wires 904, 1012, and 1013 are formed on the surface of the insulator layer 902. A protection layer 905 is formed on upper surfaces of the bonding pad 903 and the wires 904, 1012, and 1013. The terminal portions 1011a and 1011b are connected to the n-type silicon layer 1011. The wires 1012 and 1013 are connected to a ground potential. The p-type silicon substrate 901 is isolated from the n-type silicon layer 1011 via a depletion layer provided as a p-n junction. The p-type silicon substrate 901 is connected to a ground potential (not shown).
In the configuration shown in FIGS. 17A and 17B, an HF signal leaked from bonding pad 903 through a parasitic capacitance of the insulator layer 902 flows to the ground potential via the n-type silicon layer 1011 and the wires 1012 and 1013. The resistant component of each of n-type silicon layer and the wires 1012 and 1013 is very smaller than that of the silicon substrate 901. Consequently, the HF-signal loss is reduced in the configuration shown in FIGS. 17A and 17B.
However, as shown in FIG. 18, in the prior-art HF semiconductor devices, an increase in frequency involves an increase in the influence of a parasitic inductance Ls occurring in, for example, the wire 1012,1013. This causes the n-type silicon layer 1011 to be isolated in high frequency from the ground. As such, the HF signal leaked from the n-type silicon layer 1011 does not flow to the ground potential, but it flows to the p-type silicon substrate 901. In this case, the resistant component of the p-type silicon substrate 901 causes a loss of the signal component.